    Non-Patent Document 1: O'Regan, B., et al., “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films”, Nature, October 1991, Vol. 353, No. 6346, P. 737    Patent Document 1: JP-A 2006-265094    Patent Document 2: JP-A 2007-163638    Patent Document 3: JP-A 2007-176753    Patent Document 4: JP-A H6-340423    Patent Document 5: JP-A 2005-96059    Patent Document 1: JP-A 2003-225900
Titanium dioxide superfine particles have been widely used as additives to pigments, chemical fibers, paints, ink, pharmaceuticals, cosmetics, UV shielding agents, silicone rubbers and the like; a thin film of titanium dioxide formed on the surface of a substrate such as ceramics, plastics and the like; dielectric raw materials, catalysts, catalyst supports, adsorbents, photocatalysts, anti-bacterial agents, deodorants, optical materials, fillers, electronics materials, antibacterial tiles, self-cleaning building materials, electronic paper; antireflection films on the display surface of flat panel displays (FPD) such as liquid crystal displays (LCD), plasma displays (PDP) and electroluminescence displays (EL); and the like. Further, in titanium dioxide, an application research as a solar cell has progressed since Graetzel et al., at the Swiss Federal Institute of Technology Lausanne, reported a dye-sensitized solar cell using titanium dioxide and a ruthenium based colorant in combination (Non-patent Document 1).
Generally, titanium dioxide has three kinds of crystal structure:rutile, anatase and brookite. There is a different property in the crystal structure such that among these, in the field of using the above photocatalysts and solar cells, atanase or brookite titanium dioxide excellent in photoelectrochemical activities is more widely used than rutile titanium dioxide (Patent Document 1).
Further, upon use of titanium dioxide as a photocatalyst, for example, if its dispersibility is low, a shielding force would be strong and therefore, the use application may be limited. Since the titanium dioxide utilized in the field of solar cells often has a primary particle size of several nm to several tens nm, if the dispersibility is favorable, an effect on light scattering is small. However, titanium oxide having low dispersibility and a large aggregated particle size results in strong light scattering, and therefore deteriorates properties as a solar cell.
Recently, as a next generation display, the development and research of electric paper have been actively carried out which incorporates an advantage of paper as a display medium together with organic electroluminescence and inorganic electroluminescence. That is, electric paper using negatively charged titanium dioxide superfine particles and positively charged carbon superfine particles is expected as a rewritable display device maintaining convenience of paper as it is due to its weight saving and flexibility, as compared with the conventional panel display. Further, the electric paper can be driven at a low voltage, and moreover is excellent in energy saving properties since an image once displayed by application of a voltage can be maintained as it is without consuming electric powers (Patent Document 2).
General methods for producing titanium dioxide superfine particles are a sulfuric acid method and a chlorine method. Examples of methods for synthesizing commercially available titanium dioxide particles include a hydrothermal synthesis method, a combustion method of titanium tetrachloride, a sol-gel method and a plasma CVD method. The titanium dioxide superfine particles produced by the conventional synthesis method have, however, an infinite form, less crystallinity, wide particle size distribution, and low purity. In the case of using such titanium dioxide superfine particles, the usage is restricted, and the titanium dioxide superfine particles can be used only in an application where optical semiconductivity is not utilized, such as paintings, pigments and UV cut films.
What is required for an existing titanium dioxide material is titanium dioxide superfine particles having a small particle size, large specific surface area and favorable dispersibility.
On the other hand, examples of a method for producing titanium dioxide superfine particles in which the forms of particles are controlled include those described in Patent Documents 3 and 4. However, any examples have not sufficiently satisfied the above requirements such that titanium dioxide superfine particles having a uniform nanosized particle size are not obtained, the particle size is large, the cost is high because of using various reagents, or the like.
Hereafter, there is a high possibility that the development of the above production method leads the complication of steps and the prolongation of time required for production. All these problems are caused by a side reaction or temperature ununiformity due to photoreaction, or by a side reaction due to ununiformity of stirring in a reactor, upon obtaining titanium dioxide superfine particles. Therefore, uniformization of the particle size and unification of reaction products can be effectively and efficiently carried out by suppressing such side reactions.
As described in Patent Document 5 or Patent Document 6, there have been reported production methods using various microreactors or micromixers, and there are many advantages in the micro-devices and systems, but as the micro-flow path diameter is decreased, pressure loss is inversely proportional to the biquadrate of the flow path; that is, extremely high feeding pressure becomes necessary thus making a pump for actually feeding a fluid hardly available. In addition, there are many problems; for example, a phenomena of clogging of a flow path with a product or clogging of a micro-flow path with bubbles generated by a reaction occurs when the reaction is accompanied by separation, and therefore a microscopic space is not effective or applicable to every reaction. Actually, the reaction should be attempted by trial and error in order to select good results. If scaling up has been coped with a method of increasing the number of microreactors, that is numbering up, there is a problem that the absolute number of failure causes would increase, and when the problem of clogging actually occurs, it can be very difficult to detect a problem site such as a failure site.
In light of this situation, it is an object of the present invention to provide a method for producing titanium dioxide superfine particles obtained in a thin film fluid formed between two processing surfaces arranged to be opposite to each other to be able to approach to and separate from each other, at least one of which rotates relative to the other, wherein the reaction in the thin film fluid is highly uniform, so that monodispersed titanium dioxide superfine particles can be prepared depending on its purpose, and wherein clogging with a product does not occur due to self-dischargeability, great pressure is not necessary, and productivity is high, and to provide titanium dioxide superfine particles obtained therefrom.